1. Field of the Invention
Example embodiments of the present invention relate to semiconductor memory devices and a methods of operating the same, for example, multi-bit nonvolatile memory devices including at least one carbon nanotube channel and methods of operating the same.
2. Description of the Related Art
Semiconductor memory devices may volatile or nonvolatile memory devices. Volatile memories, such as DRAMs or the like may be used to store data in a hard disc and/or process data more rapidly when a computer is turned on. Volatile memories may be included in electronic equipment, such as computers or the like.
Non-volatile memories, in addition to being usable in computers, may also be used in mobile phones, digital cameras, and the like. Non-volatile memories may have higher processing speeds and/or may store data even when the power is turned off.
Nonvolatile memory devices may use the transition of a threshold voltage of a transistor, charge displacement or a change in resistance for recording and/or erasing data. Non-volatile memory devices using the threshold voltage transition of a transistor may include, for example, flash memory devices using a floating gate as a storage node, SONOS memory devices using a charge trap as a storage node, or the like. A non-volatile memory device using charge displacement may be, for example, a ferroelectric memory (FRAM) including a nano-crystal or a polymer. Non-volatile memory devices using resistance variation may include a magnetic random access memory (MRAM), phase-change random access memory (PRAM), resistance random access memory (RRAM) or a polymer memory. These nonvolatile memory devices may have lower integration density and/or lower capacity, which may result from limits of manufacturing process therefore.
In related art memory devices, channels may be formed by doping a semiconductor substrate formed of silicon with a dopant. A planar memory device may be formed on the semiconductor substrate on which the channel is formed. However, a three dimensional structure memory device for a multi-bit operation may not be formed.